1. Field of the Invention
The present invention relates generally to apparatus for the determination of the composition of a sample by spectrophotometry. More particularly, the invention provides a sensor assembly including one or more microlamps fabricated onto a semiconductor chip. Such a sensor assembly may find particular use in a capnometer, a medical device for measuring carbon dioxide content in the exhaled breath of a patient, especially one whose breathing is controlled by a mechanical ventilator.
2. Description of the Background Art
Carbon dioxide (CO.sub.2) is produced in the body as a normal byproduct of human metabolism. Gaseous CO.sub.2 is eliminated from the body through the lungs in exhaled breath. The concentration of CO.sub.2 carried by blood in the arteries is the primary stimulus for respiratory control. Normally, if CO.sub.2 builds up in the blood, the body's regulatory functions increase the breath rate until the CO.sub.2 concentration returns to a normal level.
In a patient breathing with the assistance of a mechanical ventilator, however, the patient's breath rate is controlled by the machine. Should CO.sub.2 begin to accumulate in the blood, the ventilator controls must be adjusted to restore the CO.sub.2 concentration to an acceptable level. Although the blood CO.sub.2 level may be measured directly, e.g., by blood gas analysis, it is usually preferable to monitor the concentration of CO.sub.2 in the patient's exhaled breath. Breath monitoring allows adjustments to be made almost immediately, compared with a delay of at least several minutes with blood-gas analysis.
Instruments for the measurement of CO.sub.2 concentration in the exhaled breath of a patient are known in the medical arts. These devices are called "capnometers," from Greek, kapnos, for "smoke." One such instrument has been available as the Hewlett-Packard Model 47210A Capnometer. This device is described in Solomon, R. J., "A Reliable, Accurate CO.sub.2 Analyzer for Medical Use," Hewlett-Packard Journal, September 1981, pp. 3-21.
The 47210A system uses spectrophotometry to measure the CO.sub.2 level in a patient's exhaled breath. Radiation from an infrared source is transmitted through a gas sample chamber through which exhaled air passes. Radiation from the source is partially absorbed as it passes through the gas sample. The degree of absorption is strongly dependant upon wavelength but for a given wavelength the degree of absorption is directly related to the concentration of CO.sub.2 present in the sample. By passing radiation of a preselected wavelength known to be strongly absorbed by CO.sub.2 through a sample tube containing air exhaled by the patient, and by determining the amount of radiation absorbed by the sample, the concentration of CO.sub.2 in the patient's exhaled breath can be conveniently determined with a high degree of precision.
FIG. 1 depicts the infrared radiation source used in the Model 47210A capnometer. Infrared source 10 includes a glow bar 15 connected to a pair of current leads 17 and 18. The glow bar is disposed in front of an ellipsoidal mirror 20 inside a transistor can 23. Glow bar 15 is formed of a resistive ceramic-metal composite (cermet) material. When current flows through the glow bar, electrical resistance heats the glow bar, causing it to emit infrared radiation across a broad band of wavelengths. This radiation is focused and directed outwardly from source 10, through a sapphire window 25 by mirror 20.
From infrared source 10, the infrared radiation is directed through a sample chamber 30 as depicted in FIG. 2. The sample chamber is a tube through which passes breath exhaled by the ventilated patient. Sample chamber 30 should be located as near as possible to the patient so that measurement of CO.sub.2 in the sample can be made immediately as the breath leaves the patient. For this reason, the sample chamber is preferably formed as a part of an airway adapter 32, through which the patient is ventilated.
The sample chamber 30 may, however, also be located away from the patient, with a sample line going to the patient's airway. The disadvantage of such a "sidestream" configuration is that there is typically a delay of several seconds and some smoothing of the waveforms, but neither of these problems is significant in most clinical settings.
A rotating filter wheel 33 is disposed between sample chamber 30 and an infrared detector 35. Filter wheel 33, driven by a motor drive (not shown), serves two important functions. First, the filter wheel includes three individual elements through which radiation is transmitted. Two of these elements are sealed gas cells 37 and 38. One cell contains a reference gas sample containing a known concentration of CO.sub.2 ; the other cell holds only nitrogen. A third element is an opening 40, a hole in the filter wheel, through which radiation passes unimpeded. Finally, the material of the filter wheel in an area 42 opposite opening 40 is opaque to radiation from source 10.
Infrared source 10, filter wheel 33, and detector 35 are housed inside a sensor housing 44, which clips onto airway adapter 32 over sample chamber 30. The concentration of CO.sub.2 within sample chamber 30 is computed by comparing the intensity of radiation impinging on detector 35 when each of wheel elements 37, 38, 40, and 42 lies between source 10 and the detector. The filter wheel in the 47210A unit rotates at 2400 rpm. Thus, forty measurements per second can be taken through a given element (open cell, reference cell, or opaque region).
The second function of the filter wheel is to serve as a mechanical shutter to "chop" the radiation into discrete on and off periods. The detector is exposed to radiation from the source only during the time when a transmissive filter element is passing in front of the source. At other times, the radiation is prevented from reaching the detector.
The "off" periods during which radiation is blocked allow the detector to settle to a zero intensity rest state. If the detector were exposed to radiation continually, the measurements would be subject to errors induced by electronic "drift" in the detector and the system electronics. Conceivably, off periods could be provided simply by switching off the current flowing through the glow bar. However, the relatively high thermal mass (mass times specific heat) of the glow bar means that a relatively long period of time would be required for the glow bar to cool. The glow bar could certainly not be switched on and off nearly as rapidly as is provided by the rotating filter wheel.
While capnometers such as the Model 47210A described above have found widespread use in the medical industry, further improvements are possible. For example, it would be desirable if the size and weight of the sensor assembly could be reduced so that placement of the sensor assembly near the patient would be more convenient. First, there is less unwanted smoothing of the signal waveforms when the instrument has a smaller sample volume (dead volume); although this problem is not as severe for adult patients as for children, it is a critical problem for neonatal patients. Second, conventional capnometers such as the Model 47210A are so heavy that they can tear the breathing tube out of neonatal patients.
It would also be desirable if the rotating filter wheel and its associated drive motor could be eliminated. This would further reduce the weight and bulk of the sensor assembly and enhance the simplicity and reliability of the entire system.